Earthquakes induced by injection wells

Researchers at UC Santa Cruz found that bigger and more distant earthquakes can occur when injecting fluid into sedimentary rock than into the underlying basement rock.

The evidence from this study reveal that the current regulations concerning hydraulic fracturing, wastewater disposal and geothermal wells may need severe revision. Emily Brodsky, professor of Earth and planetary sciences at UC Santa Cruz stated: "The current advice is to preferentially inject into the sedimentary sequence as a theoretically safer alternative to the basement rock. This is problematic." By analyzing data from earthquake incidents associated with injection wells, scientists showed that even a single injection can provoke seismic events at a radius of more than 6 miles.

Thomas Goebel, Postdoctoral researcher, commented on the fact that injections into sedimentary rocks induce more hazardous temblors than those into the underlying basement rock. "It's not that the basement rock is safe, because there is still the possibility of encountering a fault in the basement rock that can cause a large earthquake, but the probability is reduced because the spatial footprint is smaller," he said.

Injections into the basement rock provoke seismic incidents in a compact cluster around the well whereas injections associated with sedimentary rock cause earthquakes that decline slowly and occur at greater distances. Seismicity observed in the basement rock can be attributed to increased fluid pressure that causes fault slips. However, the mechanism seen into sedimentary rock is different and results from "poroelastic coupling," regulating the tension of the rock to transmit fluid stresses into the solid rock matrix. "When you inject water into the ground, it pushes on the surrounding rock and creates elastic stress in the rock, which can put pressure on faults at a distance without putting water into those faults. So if poroelasticity is dominant, you end up with a larger footprint because it's loading neighboring faults beyond the area of increased pore pressure," Brodsky explains.

The findings of the study explain the high seismic intensity in Oklahoma (a region that has many injection sites in oil and gas fields) which has seen an abrupt rise in earthquakes since 2010.